An important component of hydrocarbon exploration is to infer rock properties (lithology type, porosity, volume of shale, and water saturation) from seismic amplitude based inversion (Avseth et al., 2005). On inverting seismic data for Acoustic Impedance (AI) and Pwave velocity-Swave velocity ratio (Vp/Vs), qualitative inference about rock properties at the prospects can be achieved by plotting the inverted seismic data in the Petro-Elastic Model (PEM) grid also known as Rock Physics Template (Avseth et al., 2005). Petro-elastic model grids are built using semi-empirical rock physics models that are calibrated with analog well data. Analog well data is data from wells that have already been drilled in areas believed to have analogous rock properties to the present area of exploration. For example, analog well data may come from an area known to have a similar depositional history (e.g., fluvial) as the present area of exploration.
Petro-elastic model grids are commonly built using rock physics models in the elastic property domain (e.g., crossplot of AI Vs. Vp/Vs). An important step for reliable inference of rock properties using such PEM grids is calibration to analog well data. This is achieved by heuristically refining modeling parameters of the semi-empirical relationships. The PEM grid is used as template for inferring rock properties from the inverted seismic data (Avseth et al., 2005). Bayesian probability based inference is also commonly used for quantitative inversion (US 20090192718 A1).
Current PEM grids are lacking in the following ways:
1) Starting point of current PEM grids are rock physics models. Calibration to analog well data commonly requires significant tweaking of model parameters. The final choice of model parameters is often arbitrary and not driven by any underlying geological processes.2) PEM grids are built in the elastic property domain (e.g. P-impedance and Vp/Vs ratio) and requires seismic amplitude data to be inverted for Vp, Vs, and Density. Seismic inversion is a non-trivial process. The first step in commonly used model-based inversion is building an initial layered earth model (background model) with Vp, Vs, and Density values derived from well data for the depth interval to be inverted. In frontier basins with limited well control, biased choice of the background model may lead to un-realistic seismic inversion results.3) Input to PEM grid points (i.e., Vp, Vs, density) for different porosity values are based on semi-empirical rock physics models. They often fail to account for the underlying geological complexity and local variability of elastic properties as function of rock texture and porosity generation or reduction mechanism leading to an overly simplistic PEM grid.
There exists a need for improved petro-elastic modeling.